Ocean Alkalinity Enhancement (OAE) is a climate intervention strategy that involves adding alkaline substances to the ocean’s surface. Its goals are to increase the ocean’s absorption of atmospheric carbon dioxide (CO2) and counteract ocean acidification. This approach accelerates a natural geological process where the weathering of rocks on land releases alkaline minerals into the sea. OAE proposes to speed up this process significantly to address the rapid, human-caused increase in atmospheric CO2, potentially storing vast amounts of carbon for long durations.
Understanding Ocean Alkalinity and Its Significance
Ocean alkalinity is a measure of seawater’s capacity to neutralize acids, which helps maintain a stable pH balance. The primary contributors to this buffering capacity are bicarbonate and carbonate ions. These ions absorb the hydrogen ions that make a solution more acidic.
The ocean absorbs about a quarter of human CO2 emissions, disrupting this natural balance. When CO2 dissolves in seawater, it forms carbonic acid, which releases hydrogen ions that are neutralized by carbonate ions. As more CO2 enters the ocean, this process consumes carbonate ions faster than they are replenished, reducing the water’s alkalinity and causing its pH to fall in a process known as ocean acidification.
This decline in alkalinity and pH affects many marine organisms. Corals, shellfish, and some plankton build their shells and skeletons from calcium carbonate. As water becomes more acidic and carbonate ions become scarcer, it is more difficult for these calcifying organisms to build their structures. Increasing ocean alkalinity could support the resilience of these marine ecosystems in the face of continuing climate change.
Proposed Techniques for Ocean Alkalinity Enhancement
One proposed method is enhanced weathering, which involves grinding large quantities of alkaline minerals and distributing them in the ocean. Rocks rich in silicate minerals, such as olivine and basalt, are prime candidates. After being crushed to a fine powder to maximize surface area, these minerals can be spread on beaches or dispersed from ships.
Another technique is the direct addition of manufactured alkaline substances. This includes products like quicklime (calcium oxide) or slaked lime (calcium hydroxide), which dissolve quickly in seawater. Some proposals consider using industrial byproducts with high alkaline content, such as steel slag or cement kiln dust, which would require significant mining, production, and large-scale distribution infrastructure.
A more technologically advanced approach uses electrochemistry to alter seawater chemistry. In these systems, an electric current separates seawater into acidic and alkaline streams. The alkaline stream is discharged back into the ocean, increasing its buffering capacity, while the acidic co-product could be used to accelerate the dissolution of alkaline rocks.
Potential for Carbon Dioxide Removal and Buffering Ocean Acidification
Adding alkalinity to seawater enhances the absorption of atmospheric CO2. It triggers chemical reactions that convert dissolved CO2 into stable bicarbonate and carbonate ions. This shift lowers the concentration of dissolved CO2 in the surface water. To restore equilibrium, the ocean then draws more CO2 from the atmosphere, making the ocean a more efficient carbon sink.
This process offers a way to lock away atmospheric carbon for long periods. Once converted into bicarbonate, the carbon is stable and can remain dissolved in the ocean for thousands of years. The scale of CO2 removal is limited by the amount of alkaline material that can be sourced and distributed.
The addition of alkalinity also directly addresses ocean acidification. The process consumes hydrogen ions and increases the concentration of carbonate ions, both of which raise the ocean’s pH and make it less acidic. This change improves the chemical conditions for calcifying organisms, helping to protect vulnerable marine ecosystems from the corrosive effects of rising CO2 levels.
Ecological and Environmental Considerations
The ecological consequences of OAE are complex. By increasing the availability of carbonate ions and raising the pH, OAE could directly benefit calcifying organisms threatened by ocean acidification. This could help preserve coral reefs and support shellfish populations, which are foundational to many marine food webs.
However, there are significant risks. Adding large quantities of minerals could increase water turbidity, harming light-dependent organisms like phytoplankton and seagrasses. The minerals are also not pure; olivine, for example, contains nickel and silica. The release of these trace metals could have toxic effects or act as a fertilizer, leading to unpredictable shifts in the marine food web.
Deployment methods also carry risks. Spreading fine mineral dust could create localized areas of extremely high alkalinity and pH, harming organisms in the initial plume. Undissolved material sinking to the seafloor could also smother benthic ecosystems. There are knowledge gaps regarding how much alkalinity can be added before negative consequences, such as the unwanted precipitation of minerals, occur.
Current Status of Research and Development
OAE is in the early stages of research, with most work confined to lab experiments and computer modeling. These studies help determine how quickly different minerals dissolve and how much alkalinity can be added safely. This research aims to predict potential outcomes and avoid side effects like unintentional mineral precipitation.
A few studies use mesocosms, which are large containers of seawater, to simulate ocean conditions and study the effects on small communities of organisms. Field trials are very limited and small-scale, focusing on how alkaline materials disperse in coastal environments. A significant gap remains between these small experiments and what is required for large-scale deployment.
Before OAE can be considered viable, several hurdles must be overcome. A primary challenge is developing reliable methods for monitoring, reporting, and verification (MRV) to track CO2 removal and environmental side effects. The cost, energy requirements, and scalability of distributing vast quantities of materials are also major concerns. Finally, establishing clear governance and gaining public acceptance are necessary for future implementation.